java - 使用 RxJava Observables 组合成并行下载图像的 Map

Question

我正在尝试并行下载图像列表，并将它们组合到 map 中。

起初我尝试制作这样的 Observable:

Observable<Map<Integer, Bitmap>> getImages(final List<Activity> activities) {
    return Observable.create(new Observable.OnSubscribe<Map<Integer, Bitmap>>() {
        @Override
        public void call(Subscriber<? super Map<Integer, Bitmap>> subscriber) {
            try {
                Map<Integer, Bitmap> result = new HashMap<Integer, Bitmap>();
                for (Activity act : activities) {
                    result.put(act.getId(), downloadImage(act.getImage()));
                }
                subscriber.onNext(result);
                subscriber.onCompleted();
            } catch (Exception e) {
                subscriber.onError(e);
            }
        }
    });
}

这可行，但这不是我想要的。因为图像是按顺序下载的。我认为 for 循环在 rxjava 中不太好。所以我创建了这些可观察对象:

    Observable<Bitmap> getImage(final Activity activity) {
    return Observable.create(new Observable.OnSubscribe<Bitmap>() {
        @Override
        public void call(Subscriber<? super Bitmap> subscriber) {
            try {
                subscriber.onNext(downloadImage(activity.getImage()));
                subscriber.onCompleted();
            } catch (Exception e) {
                subscriber.onError(e);
            }
        }
    });
}

Observable<Map<Integer, Bitmap>> getImages(final List<Activity> activities) {
    return Observable
        .from(activities)
        .flatMap(new Func1<Activity, Observable<Bitmap>>() {
            @Override
            public Observable<Bitmap> call(Activity activity) {
                return getImage(activity);
            }
        })
        .toMap(new Func1<Bitmap, Integer>() {
            @Override
            public Integer call(Bitmap bitmap) {
                return 1; // How am I supposed to get the activity.getId()?
            }
        });
}

所以我制作了一个 Observable 来获取单个图像，尝试使用 flatMap 将它们组合到第二个图像中。这可行，但仍然有 2 个问题:

1. 当我执行 toMap() 时，如何检索正确的 id 来用作 map 的键？我想为此使用 Activity 对象。
2. 遗憾的是，下载仍在按顺序而非并行处理。我该如何解决这个问题？

Answer 1

创建一个包含位图和 Activity 的包装类。说ActivityBitmap 。替换getImage与 getActivityBitmap :

Observable<ActivityBitmap> getActivityBitmap(final Activity activity) {
    return Observable.create(new Observable.OnSubscribe<ActivityBitmap>() {
        @Override
        public void call(Subscriber<? super ActivityBitmap> subscriber) {
            try {
                subscriber.onNext(new ActivityBitmap(activity, downloadImage(activity.getImage())));
                subscriber.onCompleted();
            } catch (Exception e) {
                subscriber.onError(e);
            }
        }
    });
}

并像下面这样调用它。请注意，要获得异步下载，请使用 subscribeOn在 flatMap 。构建Map<Integer,Bitmap>最后你使用不同的重载 toMap它允许您指定键和值。

Observable<Map<Integer, Bitmap>> getImages(final List<Activity> activities) {
    return Observable
        .from(activities)
        .flatMap(new Func1<Activity, Observable<ActivityBitmap>>() {
            @Override
            public Observable<ActivityBitmap> call(Activity activity) {
                return getActivityBitmap(activity).subscribeOn(Schedulers.io());
            }
        })
        .toMap(new Func1<ActivityBitmap, Integer>() {
            @Override
            public Integer call(ActivityBitmap activityBitmap) {
                return activityBitmap.getActivity().getId();
            }
        },new Func1<ActivityBitmap, Bitmap>() {
            @Override
            public Integer call(ActivityBitmap activityBitmap) {
                return activityBitmap.getBitmap();
            }
        });
}